Low output noise switched mode power supply with shieldless transformers

ABSTRACT

A switched mode power supply which uses a shieldless transformer ( 11 ) in the dc to dc converter. Common mode switching noise is reduced by connecting two capacitors ( 17, 18 ) in series between the transformer primary ( 12 ) and secondary ( 13 ) circuits and connecting an inductor ( 19 ) between the junction of the two capacitors ( 17, 18 ) and chassis earth. The values of the capacitors ( 17, 18 ) and inductor ( 19 ) are selected such that the capacitors ( 17, 18 ) present a low impedance to noise signals and the inductor ( 19 ) presents a high impedance to noise signals.

RELATED APPLICATIONS

The present application is a 35 U.S.C. §371 national phase applicationof PCT International Application No. PCT/NZ02/00071, having aninternational filing date of 12 Apr. 2002, and claiming priority to NewZealand Patent Application No. 511144, filed 12 Apr. 2001, thedisclosure of which is incorporated herein by reference in theirentireties. The above PCT International Application was published in theEnglish language and has International Publication No. WO 02/084679 A1.

TECHNICAL FIELD

This invention relates to switched mode power supplies, and inparticular to the reduction of common mode switching noise on the dcoutput

BACKGROUND ART

The heart of all switched mode power supplies (“SMPS”) is the dc to deconverter stage. While various switching and circuit topologies areavailable, a transformer effective to transfer power at the switchingfrequency is always provided for two reasons. The first is to make useof the transformer primary to secondary turns ratio to secure therequired output voltage at maximum efficiency, and the second is tosecure isolation between the input and output of the supply.

The converter transformer may have one or more primary windings whichmay be connected so as to vary the effective transformer turns ratio asa function of input frequency as disclosed in applicant's U.S. Pat. No.5,907,236. The transformer secondary may have a single winding or becentre tapped

In many SMPS applications switching noise on the de output must beminimised. Regulations limiting “de conducted enissions” are beingpromulgated in Europe. Switching noise is conveyed to the de outputthrough the converter transformer. One technique for minimising commonmode noise transfer is to screen the primary and secondary windings. Athird screen connected to chassis earth may be included between the twowindings to minimise input-output capacitance. This is shown in FIG. 1.This ensures that noise signals are returned to the noise sourcesthrough the shortest possible path, rather than appear between thetransformer secondary winding and chassis earth. This technique meansthe transformers are expensive. Apart from the additional screeningmaterials the construction of shielded transformers is labour intensiveand reliability may be compromised if the terminations between thegrounding leads and the shields are not of high quality and have thepotential to make electrical contact with any of the windings. Shieldsalso result in reduced efficiency, due to eddy current losses.

An alternative technique for reducing common mode noise on the do supplywould be to connect a capacitor of suitable value between thetransformer secondary circuit and the transformer primary circuit. Thiswould form a capacitance divider with the transformer primary tosecondary capacitance and so reduce both the voltage and impedance ofthe noise source. However the provision of such a capacitor greatlyincreases the input-to-output capacitance of the supply and in someapplications will exceed the maximum input-to-output capacitanceallowable. Further, in many cases the input-to-output capacitance of thetransformer alone may make desired minimums difficult to attain

SUMMARY OF INVENTION

It is therefore an object of the present invention to provide atransformer circuit for a switch mode power supply which reduces theswitching noise at the supplied dc output while overcoming theabovementioned disadvantages.

Accordingly in a first aspect the invention consists in a switched modepower supply including a do to dc converter with a transformer operableat the converter switching frequency having one or more primary windingsand one or more secondary windings, the improvement comprising:

-   -   two capacitors in series connected between the secondary        transformer circuit and primary transformer circuit,    -   and an inductor connected between the junction of said two        capacitors and chassis ear, the values of said capacitors and        inductors being selected such that said series capacitors        present a low impedance to noise signals and said inductor        presents a high impedance to noise frequencies.

In a second aspect the invention consists in a transformer circuit for adc to dc converter which reduces switching noise at the dc outputcomprising:

-   -   a transformer operable at the converter switching frequency        having one or more primary windings and a secondary winding;    -   two capacitors in series connected between the secondary        transformer circuit and primary transformer circuit,    -   and an inductor connected between the junction of said two        capacitors and chassis earth, the values of said capacitors and        inductors being selected such that said series capacitors        present a low impedance to noise signals and said inductor        presents a high impedance to noise frequencies.

In a third aspect the invention consists in a transformer with a lowdielectric medium interposed between primary and secondary windings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows, a prior art shielded transformer for a SMPS,

FIG. 2 shows a block diagram of a typical SMPS,

FIG. 3 shows in amplified form SMPS transformer circuits incorporatingthe present invention,

FIG. 4 shows a simplified circuit for a SUPS according to one preferredis embodiment of the present invention; and

FIG. 5 shows two adjacent winding layers in an SMPS transformeraccording to the third aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention resides in the transformer circuit of the dc to deconverter in a SMPS. A typical SMPS is as shown in FIG. 2(a) made up offunctional blocks 1 to 5. The AC mains is filtered by a line filter 1,then a boost converter 2 produces DC at say 420 v across reservoircapacitor 3.

A dc to dc converter 4 then provides the desired output voltage (say 24Vor 48V nominal) and isolation. The dc to dc converter 4 incorporates aswitching circuit shown diagrammatically in FIG. 2(b) to provide asquare wave to transformer circuit 7 which is shown in more detailconfigured around transformer 11 in FIG. 2(c). The switching circuitemploys four power FETs arranged in the bridge configuration shownalthough other arrangements could be used.

In the preferred embodiment the transformer primary circuit shown inFIG. 2(c) is of the type disclosed in U.S. Pat. No. 5,907,236.Conventional primary circuit configurations could alternatively be used.The transformer secondary winding is centre tapped and a conventionalfull wave rectifier used to provide the converter (and power supply) dcoutput voltage V_(out).

The principles of the present invention will be described with referenceto FIG. 3(a) where for the sake of simplicity a dc converter transformer11 is shown having a single primary winding 12 and a single secondarywinding 13. The voltage in the primary winding is a square wave at theswitching frequency, that being determined by the frequency of theswitching devices 6 switching voltage from the dc input to thetransformer primary. Because the transformer primary voltage is a squarewave, considerable noise will be present due to the presence of theharmonics of the switching frequency. In a conventional non-shieldedtransformer these harmonics will be capacitively coupled into thesecondary winding to a greater or lesser extent

The present invention effectively inserts a by-pass capacitor 14 betweenthe secondary winding 13 and the primary winding 12 to provide a returnpath for common mode noise signals present in the secondary winding 13back to the source of the noise signals (the switching devices 6) asshown in FIG. 3(a). For simplicity a capacitor is shown connected toonly one end of each of the primary and secondary windings. Referring toFIG. 3(b) input to output capacitance between the primary and secondarywindings is minimised by using two capacitors 17 and 18 in series toprovide the effective bypass capacitance with an inductor 19 connectedfrom their junction to chassis earth which has a low impedance tofrequencies well below the noise frequencies (typically above 100 k Hz).

Referring now to FIG. 4 a simplified circuit for the preferredembodiment of the invention is shown. As well as making use of the splitby-pass capacitor technique already described use is made of a commonmode choke in the line filter stage of the power supply. Capacitor C₁forms one half of the split by-pass capacitor and is connected betweenone end of the secondary winding of transformer T₁ and C₂ the other halfof the by-pass capacitor. The other end of C₂ connects to the primarywinding of T₁ through either of switching transistors Q₁ or Q₃,whichever is turned on.

A conventional common mode choke L₂ is incorporated in the AC linefilter to reduce noise emission back into the mains. However the samecommon mode choke L₂ is used to filter noise current flowing through thetransformer T₁. Capacitors C₃, C₄ and C₅ ensure that noise currentsflowing back to the noise source flow through common mode choke L₂ to beconsiderably attenuated. The noise current in the preferred embodimentis thus forced to flow from source, through transformer T₁, throughground (chassis), through choke L₂ and back to source. Suitable valuesfor capacitors C₁, C₂ arm 4.7 nF, for capacitors C₃, C₄ are 4.7 nF, andfor chokes L₁ and L₂ 2 mH.

The present invention allows inexpensive non-shielded transformers to beused in SMPSs having low de conducted emissions in place of expensiveshielded transformers previously required and without increased input tooutput capacitance. Incidental benefits include increased reliabilityand efficiency.

Capacitance coupline between input and output can be reduced byintroducing a relatively inexpensive modification to conventionalshieldless transformer construction. Referring to FIG. 5 the capacitancebetween the primary and secondary windings 21 and 22 of a dc to dcconverter transformer 20 will be a function of the value of thedielectric separating the layers of windings. Advantage is taken of thisby introducing a layer of low dielectric material 23 between thewindings. This can be teflon or nomex which each have a dielectric valueof around 2 which is lower than air, paper and other materials normallyfound between transformer windings.

Various modifications, alterations and variations that will readilyoccur to those of skill in the art are intended to be within the scopeand spirit of the invention. Accordingly, the foregoing description ofthe preferred embodiments is by way of example only and is not intendedas limiting. The scope of the invention is defined only by the followingclaims and the equivalents thereto.

1. In a switched mode power supply including a dc to dc converter with atransformer operable at the converter switching frequency having one ormore primary windings and one or more secondary windings, theimprovement comprising: two capacitors in series connected between thesecondary transformer circuit and primary transformer circuit, and aninductor connected between the junction of said two capacitors andchassis earth, the values of said capacitors and inductors beingselected such that said series capacitors present a low impedance tonoise signals and said inductor presents a high impedance to noisefrequencies.
 2. A switched mode power supply according to claim 1wherein said power supply includes an AC line filter having a commonmode choke and by-pass capacitors are connected between the AC line andchassis earth and between the transformer secondary and chassis earth tocause noise currents passing through chassis earth to be returned tosource through said common mode choke.
 3. A switched mode power supplyaccording to claim 1 wherein said two capacitors are connected betweenone end of the transformer primary winding and one end of thetransformer secondary winding and two further capacitors in series areconnected between the other end of the secondary winding and the otherend of the primary winding and the junction of said two furthercapacitors is also connected to said inductor.
 4. A transformer circuitfor a switched mode power supply for a dc to dc converter which reducesswitching noise at the dc output comprising: a transformer operable atthe converter switching frequency having one or more primary windingsand a secondary winding; two capacitors in series connected between thesecondary transformer circuit and primary transformer circuit, and aninductor connected between the junction of said two capacitors andchassis earth, the values of said capacitors and inductors beingselected such that said series capacitors present a low impedance tonoise signals and said inductor presents a high impedance to noisefrequencies.
 5. A transformer circuit for a switched mode power supplyaccording to claim 4, wherein said two capacitors are connected betweenone end of the transformer primary winding and one end of thetransformer secondary winding and two further capacitors in series areconnected between the other end of the secondary winding and the otherend of the primary winding and the junction of said two furthercapacitors is also connected to said inductor.